Travelling wave tubes



United States TRAVELLING WAVE TUBES Warren D. McBee, Levittown, N. Y., assignor to Sperry Rand Corporation, a corporation of Delaware This invention relates to travelling wave tube amplifiers, and more particularly, is concerned with an improved helix supporting structure for a travelling wave tube.

To achieve broad band operation of a travelling wave tube amplifier, one of the problems has been to achieve a broad band matched coupling between the ends of the helix of the travelling wave tube and the input and output coaxial transmission lines which couple the R. F. energy into and out of the helix. It has been proposed in the prior art that matched coupling may be achieved in two steps, namely by converting the wave on the helix from the helix mode to a TEM mode and then matching the impedance of the TEM-operating helix to the impedance of the TEM-operating coaxial line. Mode con version is achieved by surrounding the helix by an outer conductor, the ends of which are tapered so as to provide reduced spacing between the helix and the conductor at the ends of the helix. In the center region of the helix, where the spacing between the helix and outer conductor is large, propagation exists in a typical helix mode of propagation with the electric field vectors extending axially between adjacent turns of the helix. At the ends of the helix where the spacing between the outer conductor and the helix is reduced, electric field lines are said to extend between the helix and the outer conductor rather than between adjacent turns, and so propagation takes place in the TEM mode. It has been further considered that by a proper spacing between the helix and the outer conductor at the ends of the helix, impedance of the TEM-operating helix might be made equal to the characteristic impedance of the coaxial line coupled to the end of the helix.

To get a characteristic impedance of the order of 50 ohms (a characteristic impedance of standard coaxial line) requires extremely close spacing between the helix and the outer conductor, the required spacing being of the order of .1 to .2 of the diameter of the wire in the helix. It has heretofore been impractical to achieve such close spacing in a travelling wave tube. One known prior art solution is that the coaxial lines be tapered to provide a higher characteristic impedance where they join with the helix to permit wider spacing between the helix and the outer conductor. However, it has been found that with wider spacing of the helix from the outer conductor, propagation in the TEM-mode is only partially achieved. The broad band matching characteristics desired are therefore not in fact achieved in practice by known methods as taught in the prior art.

In the copending application Serial No. 426,362 filed April 29, 1954, there is described a matched coupling technique in which a strip line in the form of a helically wound flat strip is utilized to couple the wire helix to the coaxial input and output lines. The increased width of the helically wound strip gives lower impedance without reducing the spacing. While the coupling therein described gives excellent results, it is somewhat diflicult to construct and assemble.

It is the general object of this invention to avoid and overcome the foregoing and other difiiculties in and objections to the prior art practices by the provision of a travelling wave tube amplifier having an improved helix supporting and matching structure.

Another object of this invention is to provide means for coupling the helix of a travelling wave tube to a standard 50 ohm transmission line over a broad'frequency band with a minimum of reflection being introduced by the coupling means.

Another object of this invention is the provision of matched coupling into and out of the helix of a travelling wave tube by means which is mechanically rugged, yet is easily assembled and can be manufactured to close tolerances.

These and other objects of the invention which will become apparent as the description proceeds are achieved by providing in a travelling wave tube having a helix and coaxial input and output lines, improved means for supporting the helix and matching the helix to the coaxial lines. This matching and supporting means includes a pair of circular horns positioned at opposite ends of and concentrically with the helix, and joined at their throat portions to the respective outer conductors of the input and output coaxial lines. The throat portion of each horn and the helix have proper relative diameters so as to propagate energy in the TEM mode and have the required characteristic impedance for matching the coaxial lines. Spaced bores in the inner surface or" the horn accept and support a plurality of ceramic rods which extend between the two horns and support the helix over the length thereof. The rods also support the helix within the horns so as to insulate the helix from the walls of the horns and provide the desired close spacing between the helix and the horns.

For a better understanding of the invention reference should be had to the accompanying drawing, wherein:

Fig. l is a sectional view of a travelling wave tube incorporating the features of the present invention; and

Fig. 2 is a fragmentary cross-sectional view taken substantially on the line 22 of Fig. 1.

Referring to Fig. l in particular, the travelling wave tube as there illustrated includes an evacuated envelope indicated generally at 10, which includes an elongated cylindrical shell 12. Supported within the envelope 10 is a first pole piece and focussing electrode 14. An electron gun assembly, a portion of which is shown and indicated at 16, directs a stream of electron through an aperture 18 in the pole piece 14 along the longitudinal axis of the travelling wave tube.

At the opposite end of the travelling wave tube is a second pole piece 20 secured within the envelope It the pole piece 20 having an aperture 22 therein for passage of the electron stream. The aperture 22 has an enlarged portion 24 for receiving a collector electrode 26. The collector electrode 26 is supported by an end plate 28 forming part of the evacuated envelope 10. For ease of assembly, a collar 30 is provided to support the end plate 28 from the shell 12, all joints between the shell 12, collar 30, and end plate 23 being brazed after assembly to render them vacuum tight.

Secured to one face of the pole piece 14 is a metallic cylindrical member 32 which projects into the region defined by the cylindrical shell 12. The member 32 has a tapered opening therein which is axially aligned with the aperture 18 along the longitudinal axis of the travelling wave tube. The member 32 with its tapered aperture may be characterized as a horn having a throat portion 35 and a mouth portion 37. A similar horn member 38 is se' cured to the pole piece 20, the horn member 33 having a flaring mouth portion 4%) and a throat portion 42.

The'inner conductor 48 extends'through an aperture 52 in the hornmember 32, the inner conductor 48 being connectedto' the end of the wire helix 44 within the throat portion 36 of the horn member 32. J

Similarly, a'coaxial line section, indicated at 54, having an inner conductor 51 and an outer conductor 56, extends throughthe cylindrical shell 12, the outer conductor 56 joining the'horn member 38 in a T-junction and the inner conductor 51 passing through an aperture 60 and the horn member 38 and being connected tothe end of the helix 44 for coupling energy out of the helix. .Both the input and output coaxial line sections have a standard characteristic impedance of 50 ohms. 7

To obtain a match between the wire helix 44 and the 50 ohm input and output coaxial lines 46 and 54 requires that propagation along the wire helix be converted from the normal helix mode, Where the electric field lines extend between adjacent turns of 'the helix, to the TEM mode, where the electric field lines substantially all terminate on. the adjacent horn members. Furthermore, the characteristic impedance of the TEM mode transmission line formed by the helix in close proximity to either of the horn members must be equal to the characteristic impedance of the coaxial lines, namely, of the order of 50 ohms.

To meet the above requirements, it has been found that the distance between adjacent turns of the helix should be of the order of three times the distance (indicated as h in Fig. 2) between the inner surface of the horn memher and the center of the helix wire. Spacing between the inner surface of thehorn member 32 and the center of the helix wire in turn is determined by the'desired characteristic impedance, which is expressed by the following formula:

where a. is the diameter of the helix wire and h is the distance between the center of the helix Wire and the inner surface of the horn member 32.

Suitable dimensions for a high power tube which have been found to provide a match between a wire helix and a 50 chin-coaxial line are as follows:

Mean helix diameter--.900 inch Pitch distance.25O inch.

Helix wire diameter.080 inch Wire center to horn distance h).05 6 inch.

it Will be noted from the above-cited dimensions that the spacing between the helix and the horn member is quite small, namely, of the :order of .016 inch, and if the travelling wave tube is designed for operation at higher operating frequencies, the spacing becomes even smaller. Furthermore, the tolerancesbecome rather critical inasmuch as there is substantially a linear change in impedance with change in the spacing. In accordance with the present invention, this close and accurate spacing between the helix wire and the horn members is maintained by a plurality of ceramic support rods 62, preferably three in number. These rods extend between and are supported by the horn members 32 and 38, the ends of the rods 62 being positioned in holes or bores 63 accurately formed in the horn members 32. These bores are centered on a circle, the diameter of which is equal to the outer diameter of the helical conductor plus the diameter of the rods.

Since the diameter of the throat portions 36 and 42 are larger than the outer diameter of the helix 44, the bores longitudinally intersect the throat portions in the horn members, so that the rods, when in position in the bores, project into the throat portions 36 and 42 the exact required distance to maintain the proper spacing between the helix and the horn members 32 and 38. This is particularly shown in the cross-sectional view of Fig. 2.

The rods 62 are made of ceramic material which is preferably coated or otherwise treated to make them attenuators as to the ultra-high frequency signals which are developed on the outside of the helix 44, as is conventional in travelling wave tube amplifiers. A suitable clip member 70 may be positioned at spaced points along the extent of the rods to clamp the rods and wire helix tightly together.

From the above description, it will be recognized that the various objects of the invention have been achieved by the present invention by the utilization of ceramic support rods for maintaining close spacingbetween-the helix and the horn members asabove described. The close spacing and limitedtolerances plus the fact that-the entire structure is mounted within an evacuated area puts serious limitations on the helix supporting and matching structure. Close spacing between the helix and the horn members must be accurately maintained, yet the helix must be insulated from the horn members, and any-material used for maintaining this spacing must be suitable for'useWithin a high vacuum system at elevated temperatures. All .these requirements are adequately met by the structure stream of electrons along the longitudinal aided the helix, an evacuated envelope enclosing the helix and said means, a conductive circular horn positioned at each end of said helix, each horn having a throat portion of inner diameter slightly larger than the diameter of said helix, the mouths of the horns being directed toward each other, each horn having a plurality of holes extending parallel to the axis of revolution of the horns and providing longitudinal openings in the inner surface of the respective horns, and a plurality of non-conductive rods of equal diameter having their ends inserted in said holes whereby the rods are supported between the horns, the holes in each horn having their centers positioned on a circle with its center on the longitudinal axis of said helix, the diameter of the circle being equal to the outer diameter of said helix plus the diameter of one of the rods, the rods projecting axially into the-throat portion of the horns with the peripheral surface of the rods extending radially into the throat portion through said longitudinal openings, whereby said helix 'is supported in accurately spaced relationship from the inner surface of the horns by the rods, and coaxial line input and output transmission line sections extending at right angles to the longitudinal axis of said helix'with the outer andinner conductors of said coaxial line input section being connected to the throat portion of one horn and the end of said helix therewithin,iespectivelyQthe outer and inner conductors of'said coaxial line output section being connected to the throat portion :of -.the other of said horns and the end of said helix therewithin, respectively.

2. A travelling wave tube comprising a helical conductor, means for projecting a stream of electrons along the longitudinal axis of the helical conductor, an evacuated envelope enclosing said helical conductor and said means, a conductive circular horn having a throat portion of inner diameter slightly larger than the diameter of the helical conductor positioned at each end of the helical conductor, each horn having a plurality of holes extending parallel to the axis of revolution of the horns and providing openings in the inner surface of the respective horns, a plurality of non-conductive rods of equal diameter having their ends inserted in said holes whereby the rods are supported between the horns, the holes in each horn having their centers positioned on a circle with its center on the longitudinal axis of the helical conductor, the diameter of the circle being equal to the outer diameter of the helical conductor plus the diameter of one of the rods, the rods projecting axially into the throat portion of the horns with the peripheral surfaces of the rods extending radially into the throat portion through said longitudinal openings, whereby the helical conductor is supported in accurately spaced relationship from the inner surface of the horns by the rods, and coaxial line input and output transmission line sections coupled to the ends of the helical conductor.

3. In a travelling wave tube having a wire helix and coaxial line section, means for coupling the helix to the coaxial line section including a conductive horn member having a tapered opening therethrough defining a substantially cylindrical throat portion of slightly larger diameter than the outer diameter of the helix and a flaring mouth portion, the helix extending into and through the mouth portion into the throat portion of the horn member, the coaxial line section forming a T-junction with the horn member with the inner conductor of the coaxial line section extending into the throat portion and joining the end of the helix, and means for supporting the helix in closely spaced concentric relation with the throat portion of the horn member comprising a plurality of ceramic rods, the horn member having a plurality of holes extending parallel to the axis of the helix for receiving the rods, the holes being radially positioned about the axis of the helix so as to position the rods in engagement with the helix within the throat portion of the horn member, whereby the helix is supported and accurately positioned within the horn member, a small fraction of the circumference of each of said rods protruding from its hole into its horn member into the area enclosed by said throat portion of said horn member to thereby provide said closely spaced concentric relation between said helix and each throat portion.

4. Means for coupling a helix to a coaxial line section, including a conductive horn member having a tapered opening therethrough defining a substantially cylindrical throat portion of slightly larger diameter than the outer diameter of the helix and a flaring mouth portion, the helix extending into and through the mouth portion into the throat portion of the horn member, the inner conductor of the coaxial line section extending into the throat portion and joining the end of the helix, and means for supporting the helix in closely spaced concentric relation with the throat portion of the horn member comprising a plurality of non-conducting rods, the horn member having a plurality of holes extending parallel to the axis of the helix for receiving the rods, the holes being radially positioned about the axis of the helix so as to position the rods in engagement with the helix within the throat portion of the horn member, whereby the helix is supported and accurately positioned within the horn member, a small fraction of the circum ference of each of said rods protruding from its hole in said horn member into the area enclosed by said throat member of said horn member to thereby provide said closely spaced concentric relation between said helix and each throat portion.

References Cited in the file of this patent UNITED STATES PATENTS Bryant et al. Nov. 20, 

